The present invention relates to an ultrasonic motor. More particularly, the present invention pertains to an ultrasonic motor that maintains a constant pressing force by a rotor acting on a stator.
A typical ultrasonic motor has a stator and a rotor accommodated in a motor housing. The stator includes a piezoelectric element and is secured to the housing. The rotor contacts the stator with a lining member attached to the rotor in between. The rotor is pressed against the stator by a pressing member. A rotary shaft, which is supported by the housing, is coupled to the rotor to integrally rotate with the rotor. When provided with high frequency voltage, the piezoelectric element vibrates. The vibration of the element produces progressive waves between the stator and the rotor. The waves rotate the rotor.
The pressing member includes, for example, a supporting plate and a disk spring. The supporting plate is secured to the rotary shaft and is located next to the rotor at the opposite side of the stator. The disk spring is elastically deformed and is located between the supporting plate and the rotor. Thus, the rotor is pressed against the stator by the urging force of the disk spring.
The rotation characteristics of the ultrasonic motor are influenced by the urging force that presses the rotor against the stator. It is therefore necessary to properly adjust the urging force to obtain preferred rotation characteristics.
In a prior art motor, an adjustment washer having a predetermined thickness is placed between the rotor and the disk spring when installing the supporting plate and the disk spring. Thereafter, the urging force of the disk spring that presses the rotor against the stator is measured by a pressure gauge. If the measured force is not appropriate, the supporting plate and the disk spring are temporarily disassembled from the motor, and the adjustment washer is replaced with another washer having a different thickness. In this manner, the force that presses the rotor against the stator is adjusted.
The rotor is pressed against the stator with the lining member in between. That is, the rotor is separated from the stator by a distance corresponding to the thickness of the lining member. However, operation of the motor wears, or thins, the lining member. The rotor, which is pressed by the disk spring, approaches the stator by a distance corresponding to the wear of the lining member. This alters the flexion amount of the disk spring thereby changing the urging force of the disk spring. Accordingly, the force of the disk spring pressing the rotor against the stator changes with time.
In the prior art motor, a disk spring is assembled without considering future changes of its urging force due to wear of the lining member. Thus, even if the force of the rotor acting on the stator is appropriate when assembling the motor, the urging force of the disk spring will eventually be affected by wear of the lining member. As a result, the force urging the rotor against the stator will change. The rotation characteristics of the motor therefore depart from the factory settings.
Accordingly, it is an objective of the present invention to provide an ultrasonic motor that maintains the force pressing the rotor against the stator at a substantially constant value over time.
To achieve the foregoing and other objectives and in accordance with the purpose of the present invention, an ultrasonic motor is provided. The motor includes a stator, a rotor, a lining member and an elastic member. The stator has a piezoelectric element. The rotor faces the stator. The piezoelectric element vibrates the stator to rotate the rotor. The lining member is located between the stator and the rotor. The elastic member presses the rotor toward the stator. The force of the elastic member pressing the rotor changes in accordance with the deformation of the elastic member. The elastic member is pre-deformed during assembly such that its deformation in the axial direction of the motor is in a predetermined deformation range. The variation of the pressing force is relatively small for any deformation amount within the predetermined deformation range.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.